Merge pull request #1224 from riscv-software-src/uart-poll-less-often

For NS16550 UART, poll stdin less often

riscv/devices.h

@@ -148,6 +148,9 @@ class ns16550_t : public abstract_device_t {
   void update_interrupt(void);
   uint8_t rx_byte(void);
   void tx_byte(uint8_t val);
+
+  int backoff_counter;
+  static const int MAX_BACKOFF = 16;
 };
 
 class mmio_plugin_device_t : public abstract_device_t {

riscv/ns16550.cc

@@ -71,7 +71,7 @@
 
 ns16550_t::ns16550_t(class bus_t *bus, abstract_interrupt_controller_t *intctrl,
                      uint32_t interrupt_id, uint32_t reg_shift, uint32_t reg_io_width)
-  : bus(bus), intctrl(intctrl), interrupt_id(interrupt_id), reg_shift(reg_shift), reg_io_width(reg_io_width)
+  : bus(bus), intctrl(intctrl), interrupt_id(interrupt_id), reg_shift(reg_shift), reg_io_width(reg_io_width), backoff_counter(0)
 {
   ier = 0;
   iir = UART_IIR_NO_INT;
@@ -300,11 +300,19 @@ void ns16550_t::tick(void)
     return;
   }
 
+  if (backoff_counter > 0 && backoff_counter < MAX_BACKOFF) {
+    backoff_counter++;
+    return;
+  }
+
   int rc = canonical_terminal_t::read();
   if (rc < 0) {
+    backoff_counter = 1;
     return;
   }
 
+  backoff_counter = 0;
+
   rx_queue.push((uint8_t)rc);
   lsr |= UART_LSR_DR;
   update_interrupt();